1. Field of the Invention
This invention relates to the removal of carbonyl sulfide from an effluent stream of synthesis gas and, more particularly, to removing the carbonyl sulfide in a wet scrubber without having to introduce catalysts for the hydrolysis of the carbonyl sulfide other than those naturally occurring in the synthesis gas stream.
2. Description of the Related Art
In situations where fossil fuels are partially oxidized, otherwise known as gasification, (e.g., in a power plant or a refinery) a gaseous mixture is produced. This gaseous mixture is commonly called "synthesis gas" and will be referred to herein as such. Synthesis gas may be used as a fuel or a feedstock for the production of chemicals. When gasifying fossil fuels such as coal or other mixtures containing coal, the synthesis gas that is produced includes particulate matter such as coal ash.
The synthesis gas may also contain a variety of sulfur-containing compounds such as hydrogen sulfide, carbonyl sulfide, dimethyl sulfide, carbon disulfide, and other sulfides and disulfides, and may also include ethyl and methyl mercaptan, thiols, and other sulfur-containing compounds. If the synthesis gas stream is burned as a source of energy, the emissions therefrom are regulated by governmental standards that define the acceptable limits and chemical composition of sulfur-containing compounds that may be released into the air. These regulations, while being extremely valuable to preserving the environment in which we live, create additional expenses for those in the business providing electrical energy, in particular. These expenses are occasioned by the need to remove the sulfur-containing compounds from the synthesis gas stream so that the sulfur-containing compounds or their combustion products are not released into the environment.
One such sulfur compound that is subject to output limitations through regulation is sulfur dioxide. Sulfur dioxide (SO.sub.2) is produced when, for example, hydrogen sulfide (H.sub.2 S) or carbonyl sulfide (COS) is burned. Governmental agencies, on the federal, state, as well as local level, have currently been reducing through regulations the amount of SO.sub.2 which industrial plants may release into the air.
There are currently existing methods (i.e., acid gas removal systems) that reduce the amount of H.sub.2 S, in a synthesis gas stream. In turn, reducing the amount of H.sub.2 S directly reduces the level of SO.sub.2 that is emitted into the environment because SO.sub.2 is formed when H.sub.2 S is burned.
However, these acid gas removal systems are not effective in removing carbonyl sulfide (COS). When COS is burned it is converted into SO.sub.2 and carbon dioxide (CO.sub.2). Thus, in order to maintain lower SO.sub.2 emission levels, the amount of COS in the synthesis gas stream must be reduced before being treated in an acid gas removal system. Thus, the ever-tightening restrictions on SO.sub.2 emissions have created the need for the efficient and cost-effective methods and apparatus for removing COS from synthesis gas streams.
In order to reduce the amount of COS in the synthesis gas stream, a number of approaches have been proposed. The easiest to implement, when coal is the fuel, is simply to gasify coal that has a lower concentration of sulfur contained therein. If there is less sulfur in the coal, the amount of sulfur-containing products produced when the coal is gasified will be smaller. Because of the smaller amount of sulfur in the coal there is a lower concentration of sulfur containing products in the resulting synthesis gas. This, in turn, leads to a lower amount of COS being produced when the coal is gasified. Burning a synthesis gas with lower amounts of LOS, may yield SO.sub.2 in quantities that are below the established emission limits. In that case, no processing of the synthesis gas is required in order to remove COS.
However, the price of coal is generally inversely proportional to its sulfur content; that is, the less sulfur in the coal, the more expensive the coal. The price of coal having sufficiently low sulfur concentrations such that removal of COS is not required is extreme; and in some cases (depending on market conditions) the cost of using such coal would actually create a financial loss as compared to the return based upon the energy produced from burning the coal. Further, as the world's supply of coal is further diminished, the price of low sulfur coal will continue to rise and low sulfur coal eventually will disappear.
Realizing that industry needs to gasify coal having appreciable sulfur contents, there has been much study of how to remove COS from synthesis gas streams. However, COS is one of the must difficult sulfur compounds to remove from synthesis gas streams. It has a low boiling point similar to propane, so it is difficult to remove therefrom by fractionation. It is relatively stable toward acidic reagents and is only slowly affected by strong alkalies. Thus, the prior art methods for its removal involve complex processes and technically complex apparatuses.
Another approach taken in the prior art is the catalytic conversion of COS to H.sub.2 S via the COS hydrolysis reaction according to the following reaction: EQU COS+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 S
H.sub.2 S is much more easily removed from the synthesis gas than COS by treating the H.sub.2 S in the synthesis gas steam with many common solvents. A typical process configuration utilizing COS hydrolysis is as follows.
The synthesis gas stream is output from a gasification chamber where coal is partially oxidized. On one hand, the partial oxidation of the coal provides a valuable source of energy as has been known for decades. On the other, it also produces compounds that, if not disposed of before combustion, create emissions that are unacceptable under the present emission laws.
This synthesis gas flows out of the gasification chamber through known exhaust piping. In some cases, a portion of the energy in the newly created synthesis gas, in the form of heat, is removed from the synthesis gas by a heat exchanger. The heat that is removed may be used later if desired. However, whether or not the synthesis gas is passed through a heat exchanger is optional and depends on the specific location where the process is being conducted.
Typically, synthesis gas contains excessive amounts of particulate matter. Thus, regardless of whether or not the synthesis gas is passed through a heat exchanger, the gas is then passed through a particulate removal device. The particulate matter is in the form of coal ash and the like. In order to remove this particulate matter many well-known particulate removal techniques and devices exist. For example, the device may be a dry device or a wet scrubber. Regardless of the type of device, the net effect is to remove, or at least reduce the amount of, the particulate matter from the synthesis gas. If a wet scrubber is used, water soluble-contaminants such as hydrogen chloride (HCL) are also removed from the synthesis gas stream. The particulate matter and the water-soluble contaminants should be removed in order for the hydrolysis of the COS to occur effectively, as is well known in the art. As is also well known in the art, in order for effective hydrolysis of COS to occur, the synthesis gas needs to be humidified. In the case of a wet scrubber, the scrubbing process also serves to humidify the synthesis gas. Due to the pressure in the scrubber and the temperature of the synthesis gas as it enters the scrubber, the temperature of the synthesis gas as it leaves the scrubber is typically about 300.degree.-500.degree. F.
The synthesis gas is then superheated by about 50.degree. F. in order to evaporate any entrained water in the synthesis gas. Entrained water in the synthesis gas stream can damage an expensive catalyst contained in a downstream COS hydrolysis reaction chamber (discussed below). The superheated humidified synthesis gas is then passed through a COS hydrolysis reaction chamber in order to convert the COS to hydrogen sulfide (H.sub.2 S). This hydrolysis reaction chamber contains catalysts that are required for the COS hydrolysis reaction.
The synthesis gas, now having the COS sufficiently converted so that emissions standards are not violated when the synthesis gas is burned, is then cooled to condense the water vapor therefrom. The synthesis gas must be cooled before it is passed to well known acid gas removal systems for removing the H.sub.2 S from the synthesis gas stream. Prior art acid gas removal systems typically operate at near ambient temperature (around 100.degree. F.). After removal of the H.sub.2 S, the synthesis gas may then be combusted in a steam generator (boiler), or turbine and expelled into the atmosphere. Alternatively, the synthesis gas could be used for chemical synthesis, typically after a polishing sulfur removal step.
Systems that remove COS in this manner suffer from many drawbacks. The largest of these drawbacks are the expenditures associated with them. A major cost comes from having to purchase the catalysts contained in the COS hydrolysis reaction chamber. Typical COS reaction chambers contain at least one, if not more, extremely expensive catalysts that have finite lifetimes. At present the cost of such a reaction chamber can vary from several hundred thousand dollars to numbers in the low millions of dollars. Given that these catalysts have a finite lifetime, this capital expense must be born periodically; in some cases, every four to five years. If, for example, a power plant was designed to operate for 20 years, the expense of equipping the synthesis gas treatment system with COS reaction catalysts alone could require millions of dollars for catalyst replacement.
In other prior art methods, the COS levels in a gas stream may be abated by treating the gas stream with a series of chemicals chosen such that COS is removed. An example of such a process is disclosed in U.S. Pat. No. 5,523,069 to Lin which is incorporated herein by reference. However, the methods disclosed require complex treatment of the synthesis gas and are difficult to utilize.